1. Field of the Invention
The present invention relates to a semiconductor display device which uses an organic resin film as an interlayer insulating film, and more specifically to a semiconductor display device structured which has a protective circuit formed in an input/output portion to protect internal circuits against breakage brought by high voltage load such as static electricity.
2. Description of the Related Art
In recent years, a technique of forming a TFT on a substrate has greatly progressed, and its application and development for an active matrix semiconductor display device as one of the semiconductor devices have been advanced. In particular, since a TFT using a polycrystalline semiconductor film has higher field-effect mobility (also referred to as mobility) than a conventional TFT using an amorphous semiconductor film, it enables high-speed operation. It is therefore possible to control the pixel by the driver circuit formed on the same substrate where the pixel is formed, though the pixel is conventionally controlled by a driver circuit provided outside the substrate.
A TFT consists of an active layer obtained by adding impurities to impart one conductivity type on a semiconductor film, a gate electrode, and a gate insulating film formed between the active layer and the gate electrode. Further, generally, an interlayer insulating film comprised of an insulating film is formed to cover the TFT, and the wiring to be electrically connected to the TFT on the interlayer insulating film is formed on the interlayer insulating film.
In a case where a wiring to be electrically connected to the TFT is formed on the interlayer insulating film, if the surface of the interlayer insulating film is not leveled sufficiently, the wiring will be broken or, though the wiring is not totally broken and locally thinned, the wiring resistance will increase. In addition to the wiring, if a pixel electrode is formed on the interlayer insulating film, the surface irregularities in the interlayer insulating film cause the surface irregularities in the pixel electrode and the inequality thickness of the pixel electrode. This may result in unevenness in a displayed image.
It is therefore necessary to give the interlayer insulating film enough thickness, 1 to 5 μm for example, so that the shape of the TFT does not cause the surface irregularities in the interlayer insulating film.
Films for use as the interlayer insulating film are roughly divided into insulating films formed of inorganic materials (hereafter referred to as inorganic insulating films) and insulating films formed of insulative organic resin (hereinafter referred to as organic resin films).
An inorganic insulating film is formed using vapor phase growth method such as CVD method and sputtering method. Using an inorganic insulating film as the interlayer insulating film has a drawback because it takes time to form a film thick enough to level the surface using vapor phase growth method.
On the other hand, if an organic resin film is used, the interlayer insulating film is formed by applying organic resin to a substrate on which the TFT is formed, and therefore a leveled surface is easily obtained.
The wiring to be connected to the TFT is obtained by forming a film having conductivity (hereinafter referred to as conductive film) on the interlayer insulating film in which a contact hole is opened and then etching the conductive film.
The conductive film can be etched either by wet etching or dry etching. Wet etching is isotropic etching and therefore is not adaptable to wiring pattern miniaturization if it goes beyond 3 μm. Dry etching, on the other hand, is anisotropic etching and therefore can deal with wiring pattern miniaturization.
However, a problem of dry etching is that, when the conductive film on an organic resin film serving as the interlayer insulating film is treated by dry etching, the surface of the organic resin film is roughen. With the surface of the organic resin film roughened, a flatness of the pixel electrode formed on the organic resin film is impaired and pixel display is accordingly affected.
Organic resin has high water-absorbing property and swells with water in an alkaline aqueous solution which is used in development. Therefore, a dehydration step of extracting water from the organic resin film by heat treatment has to be included after development. Despite dehydration through heat treatment, the organic resin film absorbs moisture in the adjacent films or in the air. There is a fear that the absorbed moisture corrodes over time the wiring that is in contact with the organic resin film and impairs the long-time reliability of the panel.